1. Field of the Invention
This invention relates to a process for preparing a supported semi-permeable membrane by coating a membrane-forming solution directly onto a membrane support. More particularly, this invention relates to a process for preparing a supported semi-permeable membrane which has a strong bond between the semi-permeable membrane and the support and in which the semi-permeable membrane does not substantially peel off the support during use.
2. Description of the Prior Art
In conventional semi-permeable membranes used for ultra-filtration or reverse osmosis, when such membranes are used in the form of a cast plate or cylinder, they are supported on the permeated liquid side by woven fabrics, filter papers and other porous substances, or a membrane-forming solution is directly coated onto the surface of a porous substance such as a woven fabric or the like.
When a woven fabric acting as a membrane support is used as the back lining, if a pressure is imposed from the supported side of the membrane or if a compressive vibration occurs during operation, the semi-permeable membrane is apt to separate from the support and the semi-permeable membrane, which is generally composed of a material having relatively low strength, is readily damaged and broken. When a membrane-forming solution is cast directly onto a support, the solution infiltrates into the interstices of the mesh structure of the support. The tendency of peeling of the semi-permeable membrane from the support is reduced to some extent by the thus-obtained anchoring effect or mechanical interlock effect.
In general, in semi-permeable membranes which are used over a long time period, they are likely gradually to peel from their supports owing to repetitions of compression and pressure reduction during use. Therefore, a method which comprises casting a membrane-forming solution directly onto a support provides no fundamental solution of the problem of preventing peeling of the membrane.